This invention relates to a reference voltage source for driving a current source, which reference voltage source comprises:
a first common terminal, a second common terminal, a first connection terminal, and a second connection terminal; PA1 an impedance connected between the first common terminal and the first connection terminal; PA1 a first semiconductor junction and a first resistor, connected in series between the first connection terminal and the second common terminal; PA1 a second resistor connected between the first common terminal and the second connection terminal; PA1 a second semiconductor junction connected between the second connection terminal and the second common terminal; PA1 a differential amplifier having an output and having an inverting input and a non-inverting input, wherein one of the inputs is coupled to the first connection terminal and the other input is coupled to the second connection terminal; and PA1 one of the first and second common terminals is coupled to the output of the differential amplifier and the other one is coupled to a first supply terminal.
A reference voltage source of this type is disclosed in U.S. Pat. No. 4,100,436 and is known as a band-gap reference voltage source. The impedance used therein takes the form of a resistor. The output of the differential amplifier is connected to the first common terminal and the second common terminal is connected to ground. The differential amplifier imposes a constant ratio upon the currents through the first and the second semiconductor junction. The current ratio is determined by the ratio between the resistance values of the resistance of the impedance and the second resistor. The difference between the junction voltages of the first and the second semiconductor junction, which difference has a positive temperature coefficient (TC), appears across the first resistor. Consequently, the current through the first resistor also has a positive TC. This current flows through the resistance of the impedance and produces across this resistance a voltage which also has a positive TC. The differential amplifier ensures that the voltage difference between the first and the second connection terminal is negligible, so that the voltage across the resistance of the impedance between the first connection terminal and the first common terminal is equal to the voltage across the second resistor connected between the second connection terminal and the first common terminal. The output voltage at the output of the differential amplifier is the sum of the junction voltage of the second semiconductor junction and the voltage across the second resistor. As is known, the voltage across a semiconductor junction has a negative TC. In the case of suitably selected parameters the sum of the voltages across the second resistor and the second semiconductor junction has a TC of substantially zero over a wide temperature range. This sum voltage is available for further purposes at the output of the differential amplifier.
Said U.S. Pat. No. 4,100,436 discloses a variant in which both the first and the second semiconductor junction comprise diode-connected transistors. U.S. Pat. No. 4,059,793, FIG. 2 and FIG. 3, shows a second variant, in which the first semiconductor junction is the base-emitter junction of a transistor having its collector connected to the first connection terminal and having its emitter connected to the first supply terminal via the first resistor, and in which the second semiconductor junction is the base-emitter junction of a transistor having its base coupled to the base of the first-mentioned transistor and having its collector connected to the second connection terminal. In principle, this second variant is a form of the Widlar band-gap reference published in IEEE Journal of Solid-State Circuits, Vol. SC-6, No. 1, pp. 2-7, February 1971, "New Developments in IC Voltage Regulators", FIG. 2.
Integrated circuits often require not only a thermally very stable reference voltage but also one or more temperature-stable reference currents. Such reference currents are supplied by transistors arranged as current sources, with or without an emitter series resistor. The bases of the current source transistors receive a reference, which is converted into a current. However, the magnitude of the current is also determined by the base-emitter junction voltage of the current source transistors, which voltage, as is known, has a negative TC and consequently requires a correction in order to obtain a temperature-stable current.
U.S. Pat. No. 4,816,742 reveals a solution in which the negative TC of the emitter current of the current source transistor is compensated by arranging a compensation current source with a positive TC in parallel with the emitter series resistor, resulting in a zero TC of the net collector current of the current source transistor. However, this solution is less attractive owing to the additional components and the resulting additional chip area. Indeed, each current source transistor requires a compensation transistor and, in addition, a conductor is needed to drive all these compensation transistors.
European Patent Specification 0,252,320 B1 reveals another solution, for which a resistor is connected in parallel with the second semiconductor junction. A current with a negative TC then flows through this resistor and compensates for the negative TC of the base-emitter junctions of the connected current source transistors. However, this solution is used in a reference voltage source of another type than described hereinbefore, i.e. of the Brokaw band-gap reference type. In this type the first and second semiconductor junctions are base-emitter junctions of transistors whose collectors are connected to the first and the second connection terminal and whose bases are connected to the output of the differential amplifier, the sum of the emitter currents of the transistors being formed in a common resistor.